Rendering the surface of molybdenum and tungsten compositions soft-solderable

ABSTRACT

MOLYBDENUM AND TUNGESTEN CAN BE TINIDED IN A FUSED SALT BATH. THE SURFACE SO PRODUCED ARE READILY WET BY MOLTEN SOFT SOLDER AND, THEREFORE, SUCH SURFACES CAN BE READILY SOFT-SOLDERED BY CONVENTIONAL SOFT-SOLDERING TECHNIQUES TO THEMSELVES OR TO OTHER OBJECTS WHICH CAN BE SOFT-SOLDERED.

United States Patent Office U.S. Cl. 204-68 6 Claims ABSTRACT OF THE DISCLOSURE Molybdenum and tungsten can be tinided in a fused salt bath. The surfaces so produced are readily wet by molten soft solder and, therefore, such surfaces can be readily soft-soldered by conventional soft-soldering techniques to themselves or to other objects which can be soft-soldered.

This application is a continuation-in-part of my copending application Ser. No. 593,324, filed Nov. 10, 1966, now abandoned and assigned to the same assignee as the present invention.

This invention relates to a method for metalliding a base metal composition. More particularly, this invention is concerned with a process for germaniding and tiniding a base metal composition in a fused salt bath.

I have discovered that uniform, tough, adherent germanide and tinide coating can be formed on a specific group of metals employing low current densities, that is, current densities in the range of 0.05-10 amperes/dm.

In accordance with the process of this invention, the germanium or tin metal is employed as the anode and is immersed in a fused salt bath composed essentially of a member of the class consisting of the alkali metal fluorides, mixtures thereof and mixtures of the alkali metal fluorides with calcium fluoride, strontium fluoride or barium fluoride and containing from 0.0l5 mole percent of germanium fluoride or tin fluoride. The cathode employed is the base metal upon which deposit is to be made. I have found that such a combination is an electric cell in which an electric current is generated when an electrical connection, which is external to the fused bath, is made between the base metal cathode and the germanium or tin anode. Under such conditions, the germanium or tin dissolves in the fused salt bath and germanium ions or tin ions are discharged at the surface of the base metal cathode where they form a deposit of germanium or tin which immediately diffuses into and reacts with the base metal to form a germanide or tinide coating.

A particularly novel aspect of this invention is that tungsten and molybdenum, in so far as I can determine from the literature or my own tests, can not be alloyed with tin. Despite this fact, I have found that I can diffuse tin into the surface of these two metals by my process. These tinided surfaces of molybdenum, tungsten or alloys of these two metals with each other, are readily wet by molten soft solders. Since molybdenum tungsten or molybdenum-tungsten alloys can not be soft-soldered by con-' ventional techniques, my process provides a means by which articles fabricated from such metals or alloys can be rendered solderable so that they can be joined together or other metals joined to them with soft solders using the conventional techniques used for soft soldering, generally by use of a heated soldering iron. This procedure is applicable to all those molybdenum base metal compositions and tungsten-base metal compositions which are not readily wet by soft solder, i.e., the amount of other metal or metals alloyed with the tungsten or molyb- 3,567,598 Patented Mar. 2, 1971 denum, is insufficient to render the composition softsolderable, even though these other metals themselves, may be wettable by molten soft solder.

In the specification and claims I use the terms germanide or tinide to designate any solid solution or alloys of germanium or tin with the base metal regardless of whether the base metal does or does not form an intermetallic compound with germanium or tin in definite stoichiometric proportions which can be represented by a chemical formula.

The rate of dissolution and deposition of the germanium or tin are self regulating in that the rate of deposition is equal to the rate of diffusion of the germanium or tin into the base metal cathode. The deposition rate can be decreased by inserting some resistance in the circuit. A faster rate can be obtained by impressing a limited amount of voltage into the circuit to supply additional direct current.

The alkali metal fluorides which can be used in accordance with the process of this invention include the fluorides of lithium, sodium, potassium, rubidium and cesium. However, it is preferred to employ an eutectic mixtures of sodium, potassium and lithium fluorides because some free alkali metal is produced by a displacement reaction at the higher operating temperatures and rubidium and cesium are volatilized with the obvious disadvantages. Because tin and germanium are chemically much less reactive in the fused salt baths than the other metals I have used for metalliding, volatilization of potassium from the However, it is preferred to employ an eutectic mixture of the alkali metal fluorides with calcium fluoride, strontium fluoride and barium fluoride can also be employed as a fused salt in the process of this invention.

The chemical composition of the fused salt bath is critical if good germanide and tinide coatings are to be obtained. The starting salt should be as anhydrous and as free of all impurities as is possible or should be easily dried or purified by simply heating during the fusion step. Because oxygen interferes, germaniding and tiniding must be carried out in the substantial absence of oxygen. Thus, for example, the process can be carried out in an inert gas atmosphere or in a vacuum. By the term substantial absence of oxygen it is meant that neither atmospheric oxygen nor oxides of metals are present in the fused salt bath. The best results are obtained by starting with reagent grade salts and by carrying out the process under vacuum or an inert gas atmosphere, for example, in an atmosphere of nitrogen, argon, helium, neon, krypton or xenon.

The germaniding bath is especially sensitive to the presence of oxygen or moisture. When air gets into the cell, the coulombic efiiciency drops sharply, the coating quality is poor, and a sublimate fills the towers. The sublimate is primarily GeO, a very volatile material.

It was also found that hydrogen cannot be permitted to come in contact with the salt for it reduces germanium and tin fluorides, forming HF and germanium and tin. I have sometimes found that even commercially available reagent grade salts must be purified further in order to operate satisfactorily in my process. This purification can be readily done by utilizing scrap metal articles as the cathodes and carrying out the initial germaniding and tiniding runs with or without an additional applied voltage, thereby plating out and removing from the bath those impurities which interfere with the formation of high quality coatings.

The base metals which can be germanided and tinided in accordance with the process of this invention included the metals having atomic numbers of from 27 to 29, 42 to 47 and 74 to 79 inclusive. These metals are, for example, cobalt, nickel, copper, molybdenum, technetium,

ruthenium, rhodium, palladium, silver, tungsten, rhenium, osmium, iridium, platinum and gold. Alloys of these metals with each other or alloys containing these metals as the major constituent, that is, over 50 mole percent, alloyed with other metals as a minor constituent, that is, less than 50 mole percent, can also be germanided and tinided in accordance with my process, providing the melting point of the resulting alloy is not lower than the temperature at which the fused salt bath is being operated. It is preferred that the alloy contain at least 75 mole percent of the metal and even more preferred, that the alloy contain 90 mole percent of the metal with correspondingly less of the alloying constituent.

I have also found that when the metal to be germanided or tinided is molybdenum or tungsten, it is advantageous to conduct the iding process in the absence of carbon, because carbon forms a very stable metal carbide on the surface of such base metals thereby rendering it difficult to further germanide or tinide the base metal and giving less firmly adhering deposits. I have found that carbon can be removed from the fused salt bath by operating it as a cell employing as a cathode, the metals such as molybdenum or tungsten, until the carbide coating is no longer formed on the surface of the metal.

The form of the anode is not critical. For example, I can employ as the anode pure germanium metal in the form of a rod or the germanium can be employed in the form of chips in a nickel or tungsten porous basket. Tin is used as an anode by immersing a graphite crucible filled with tin into the molten salt. When carbon particles will interfere with diffusion of the tin into a metal, such as tungsten, tightly woven metal cloths, such as molybdenum, should surround the graphite crucible.

In order to produce a reasonably fast plating rate and to insure the diffusion of the germanium or tin into the base metal to form a germanide or tinide coating, I have found it desirable to operate my process at a temperature of from about 500-1l00 C. It is usually preferred to operate at temperatures of from 600l000 C. When metalliding metals, for example tungsten and molybdenum, where diffusion is slow, I prefer to operate at temperatures of at least 700 C., and preferably at least 900 C.

The temperature at which the process of this invention is conducted determines to some extent the particular fused salt bath employed. For example, an eutectic of sodium and lithium fluorides can be employed at temperatures no lower than 675 C., because of its melting point. In the range from 600 C.1000 C., I can employ the eutectic of lithium and potassium fluorides and the eutectic lithium, sodium and potassium fluorides as the fused salt. Above 850 C., lithium fluoride can be used. Above 900 C., elimination of potassium fluoride, will reduce fogging in the vapor space due to volatilizing potassium fluoride.

When an electrical circuit is formed external to the fused salt bath by joining the germanium or tin anode to the base metal cathode by means of a conductor, an electric current will flow through the circuit without any applied electromotive force. The anode acts by dissolving in the fused salt bath to produce electrons and germanium or tin ions. The electrons flow through the external circuit formed by the conductor and the iding ions migrate through the fused salt bath to the base metal cathode to be metallided, where the electrons discharge the iding ions, forming a coating. The amount of current can be measured with an ammeter which enables one to readily calculate the amount of metal being deposited on the base metal cathode and being converted to the metallide layer. Knowing the area of the article being plated, it is possible to calculate the thickness of the metallide coating formed, thereby permitting accurate control of the process to obtain any desired thickness of the metallide layer.

Although the process operates very satisfactorily without impressing any additional electromotive force on the electrical circuit, I have found it possible to apply a small voltage when it is desired to obtain constant current densities during the reaction, and to increase the deposition rate of the germanium or tin being deposited without exceeding the diffusion rate of the iding agent into the base metal cathode. The additional E.M.F. should not exceed 1.0 volt and preferably should fall between 0.1 and 0.5 volt.

Since the diffusion rate of tin and germanium into the cathode article varies from one material to another, with temperature, and with the thickness of the coating being formed, there is always a variation in the upper limits of the current densities that may be employed. Therefore, the deposition rate of the iding agent must always be adjusted so as not to exceed the diffusion rate of the iding agent into the substrate material if high efiiciency and high quality diffusion coatings are to be obtained. The maximum current density for good tiniding or germaniding is 10 amperes/dm. when operating within the preferred temperature ranges of this disclosure. Higher current densities can sometimes be used to form coatings with germanium or tin but in addition to the formation of a metallide coating, plating of the iding agent occurs over the diffusion layer.

Very low current densities (0.010.l arnp./dm. are often employed when diffusion rates are correspondingly low, and when very dilute surface solutions or very thin coatings are desired. Often the composition of the diffusion coating can be changed by varying the current density, producing under one condition a composition suitable for one application and under another condition a composition suitable for another application. Generally, however, current densities to form good quality germanide or tinide coatings fall between 0.5 and 5 amperes per dm. for the preferred temperature ranges of this disclosure.

If an applied is used, the source, for example, a battery or other source of direct current, should be connected in series with the external circuit so that the negative terminal is connected to the external circuit, terminating at the metal being metallided and the positive terminal is connected to the external circuit terminating at the metal anode. In this way, the voltages of both sources are algebraically additive.

As will be readily apparent to those skilled in the art, measuring instruments such as voltmeters, ammeters, resistances, timers, etc., may be included in the external circuit to aid in the control of the process.

Because the tough adherent corrosion resistant properties of the germanide and tinide coatings are uniform over the entire treated area, the diffusion coating compositions prepared by my process have a Wide variety of uses. They can be used to fabricate vessels for chemical reactions, to make gears, bearings and other articles requiring hard, wear-resistant surfaces. The germanided article can also be used to develop non-reflective infrared surfaces. The tinided base metal can readily be Welded by soft solder. For example, although molybdenum and tungsten cannot be soldered by conventional techniques, I have unexpectedly found that by tiniding the surface of these base metals they can be soft soldered by using an ordinary soldering iron. Other uses will be readily apparent to those skilled in the art as well as other modifications and variations of the present invention in light of the above teachings.

EXAMPLE 1 A ternary eutectic of lithium, sodium and potassium fluorides (27.5 pounds) was charged into a Monel liner (5%" diam. x 17%" deep) that was then fitted into a steel pot (6%" diam. x 18" deep). The pot was fitted with a nickel plated steel flanged lid which contained a water channel for cooling, two ports (2%" in diameter) for glass electrode towers and two 1 ports for a thermocouple probe and a vacuum connection or gas bubbler. The whole apparatus was encased in an electrical furnace for heating. A vacuum was pulled on the cell and the salt melted. Argon was let into the cell, and with an argon flush to keep air out, 190 g. of Na GeF was added to the molten salt at 500 C.

With a germanium anode 11" sq.) immersed 2" in the salt through the anode tower and a copper strip (5" x l x 0.020") immersed in the salt through the anode tower, the following germaniding data was obtained at 600 C.

TAB LE I Volts, Current anode density, polarity amps/drn.

Time (min 0 1. 3 Current on. 1.3 Current ofi.

0 Sample out.

The sample, on being withdrawn from. the salt was bright, had a grainy appearance, and had gained 0.084 g. of a theoretical 0.135 gr. (calcd on the basis of a valence change of 2; Ge++ Ge"). The coating was 0.3 mil thick, considerably harder than copper and very flexible.

Another sample of copper was germanided at 600 C. as follows to produce a thicker coating.

The sample was also bright and grainy and had gained 1.134 g. of 2.708 g. theoretical. The coating was 3 mils thick and on X-ray examination showed high concentration of both copper and germanium on the surface. Hardness measurements showed it to be between 600800 Knoop Hardness Number. The sample exhibited marked improvement to the corrosive action of nitric acid.

EXAMPLE 2 Employing the cell of Example 1, a nickel strip was then germanided at 600 C. in accordance with the procedure of Example 1.

TABLE III Volts, Current anode density, polarity amps/din? Time (min):

0 1. 25 Current on. 1.25 Current 011.

0 Sample out.

The nickel strip was bright and smooth, and had gained 0.737 g. of a theoretical 1.35. The coating was 2 mils thick and quite hard (600700 KHN). X-ray examination showed high concentrations of both nickel and germanium on the surface, indicating that the coating was all diffusion with no plating.

6 Another nickel strip was germanided at 800 C. for 15 minutes and gave essentially the same results as above but in fifteen minutes, showing that the diffusion rate is much faster at higher temperatures.

EXAMPLE 3 Employing the procedure of Example 1, a Monel strip was germanided at 600 C. in accordance with the data in Table IV.

TABLE IV Volts, Current anode density, polarity amps/rim.

Time (minutes):

As a battery.

Do. Do.

The sample gained 0.026 g. of approximately 0.045 g. theoretical and developed a 0.2 mil coating that was bright and smooth and much harder than the original Monel surface.

EXAMPLE 4 Employing the cell and procedure described in Example 1, a molybdenum strip (3" x 1 x 0.020") was germanided as follows at 600 C. The cell run was conducted in accordance with the data set forth in the following table.

The sample gained 0.082 g. of a theoretical 0.68 g. and had a bright smooth coating 0.3 mil thick that was very hard.

EXAMPLE 5 An eutetic mixture of reagent grade lithium and potassium fluorides .(18 pounds) was melted in the same manner as described in Example 1 and to the melt was added grams of stannous fluoride. A /2" diameter graphite crucible, filled with 50 grams of tin and perforated with holes just above the level of the tin was then immersed in the salt as an anode. Using argon as an inert gas, strip of copper 4" x x was tinided as follows at 700 C.:

Volts, Current anode density, polarity amps/dmfi Time (minutes):

1. Current on. 1. 1.

0 0 0 0 Current off. 0 0 0 Sample out.

The sample came from the salt very clean and light bronze in color. It had gained 0.241 g. of a theoretical 0.271 g. (valence change of 2) and had developed a 3 mil coating which was considerably harder than the initial copper and more resistant to nitric acid.

A series of tiniding runs were made with several other metals. The runs are summarized below in Table VI.

TABLE VI Current Weight, Percent Ton1p., Tiiuo density gain, coulombio Metal Quin.) amps/din. grams ellicicucy Description of coating (1) Silver 700 15 1. 0. 036 65 0.2 rlnil coat, mat finish, smooth, slightly harder than $1 ver. 700 6 4 0. 102 93 0.5 mil coat, mat finish, bright, smooth, considerably harder than platinum. 800 1. 5 2 0. 056 100 0.2 mil coat, mat finish, smooth, considerably harder than nickel. 1,000 6 0. 7 0.260 100 1.0 mil coat, bright, smooth, moderately hard.

700 12 0 75 0.094 42 0.4 mil coat. 1, 000 60 0 50 0. 331 60 1.2111111 coat, mat finish smooth, hardcr than molybenum. 1,000 15 1. 0 0.121 45 0.3 mil coat, mat finish, smooth.

Strips of molybedenum and tungsten (4" x 1" x 0.20") tinided in accordance with the procedures set forth in Table VI were readily wetted by soft solder using either a soldering iron or a gas flame and conventional soft solder paste fluxes. Such strips when soldered together or when soldered to other metals, for example, copper, nickel and iron made strong metal bonds. Joints, having an area of one-half square inch, would support a weight of at least 150 lbs. On being pried apart, the bonds tore in the middle of the solder layer rather than releasing from the molybdenum or tungsten surface. In marked contrast, when the molybendum and tungsten strips which were not tinided, were soldered in the same way, the joints were easily separated with the bond failure being due to the solder easily releasing from the tungsten and molybdenum surfaces.

Microscopic examinations and cross sections of the tinided samples showed that tin had diffused into the molybdenum and tungsten surfaces to a limited but significant extent, rather than being a tin coating on the surface. X-ray emission spectroscopy showed that traces of nickel often accompanied the tin when the tiniding was performed in a Monel vessel but that only tin was present when the tiniding was done in high purity copper vessels. Apparently a small amount of nickel from the Monel vessel had dissolved in the fused salt bath and traces of nickel had codeposited and codii'fused with the tin. This is not undesirable insofar as the coating is concerned, if nickel can be tolerated or is desired to produce a ternary coating, since such surfaces can also be soft-soldered. However, erosion of the vessel will occur over a long period of use under such conditions. Where diffusion of of only tin is desired, a Monel vessel whose surface in contact with fused salt has been tinided by the process of this invention can be used in place of a copper vessel.

It will, of course, be apparent to those skilled in the art that modifications other than those set forth in the above examples can be employed in the process of this invention without departing from the scope thereof.

The tinided tungsten and molybdenum objects are particularly useful in the production of semiconductor devices. The tinided surface can be soldered to copper wire, or other soft-solderable surfaces, etc., to join such devices together with other electrical components. A tinided tungsten or molybdenum wire is also useful in electrode applications.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of producing a surface that can be readily soft-soldered on molybdenum, tungsten, molybdenum tungsten alloys, tungsten-base metal compositions and molybdenum-base metal compositions which are not readily wet by molten soft solder, which comprises tiniding the surface of said compositions by (1) forming an electric cell containing said metal composition as the cathode, joined through an external electrical circuit to a tin anode and a fused salt electrolyte consisting essentially of a member of the class consisting of the alkali metal fluorides, mixtures thereof and mixtures of alkali metal fluorides with calcium fluoride, strontium fluoride or barium fluoride and from 0.01-5 mole percent of tin fluoride, said electrolyte being maintained at a temperature of at least 500 C., but below the melting point of said metal composition in the substantial absence of oxygen, (2) controlling the current flowing in said electric cell so that the current density of the cathode does not exceed 10 amperes/dm. during the formation of the germanide or tinide coating, and (3) interrupting the flow of electrical current after the desired thickness of the tinide coating is formed on the base metal object.

2. The method of claim 1 wherein the metal composition is molybdenum.

3. The method of claim 1 wherein the metal composition is tungsten.

4. The process for soft-soldering to a metal composition selected from the group consisting of molybdenum, tungsten, molybdenum-tungsten alloys, tungsten-base metal compositions and molydenum-base metal compositions that are not readily wet by molten soft solder which comprises tiniding the surface of said composition by the process of claim 1 and thereafter attaching a softsolderable object to at least a portion of the tinided surface with soft solder.

5. The process of claim 4 wherein the metal composition is molybdenum.

6. The process of claim 4 wherein the metal composition is tungsten.

References Cited UNITED STATES PATENTS 3/1962 Cook 204-39 3/1958 Sibert et a1 204-39 US. Cl. X.R. 20439 

